全文获取类型
收费全文 | 2430篇 |
免费 | 195篇 |
国内免费 | 174篇 |
专业分类
电工技术 | 108篇 |
综合类 | 201篇 |
化学工业 | 687篇 |
金属工艺 | 41篇 |
机械仪表 | 115篇 |
建筑科学 | 86篇 |
矿业工程 | 9篇 |
能源动力 | 23篇 |
轻工业 | 581篇 |
水利工程 | 10篇 |
石油天然气 | 67篇 |
武器工业 | 16篇 |
无线电 | 434篇 |
一般工业技术 | 314篇 |
冶金工业 | 38篇 |
原子能技术 | 21篇 |
自动化技术 | 48篇 |
出版年
2024年 | 12篇 |
2023年 | 30篇 |
2022年 | 68篇 |
2021年 | 84篇 |
2020年 | 84篇 |
2019年 | 70篇 |
2018年 | 56篇 |
2017年 | 94篇 |
2016年 | 93篇 |
2015年 | 120篇 |
2014年 | 148篇 |
2013年 | 154篇 |
2012年 | 182篇 |
2011年 | 197篇 |
2010年 | 139篇 |
2009年 | 149篇 |
2008年 | 126篇 |
2007年 | 176篇 |
2006年 | 157篇 |
2005年 | 140篇 |
2004年 | 93篇 |
2003年 | 89篇 |
2002年 | 59篇 |
2001年 | 60篇 |
2000年 | 39篇 |
1999年 | 26篇 |
1998年 | 33篇 |
1997年 | 25篇 |
1996年 | 13篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 14篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1951年 | 1篇 |
排序方式: 共有2799条查询结果,搜索用时 15 毫秒
1.
The structure and properties of high density polyethylene (HDPE) functionalized by ultraviolet irradiation at different light intensities in air were studied by electron analysis, FTIR spectroscopy, contact angle with water, differential scanning calorimetry and mechanical properties measurement. The results show that oxygen‐containing groups such as C?O, C—O and C(?O)O were introduced onto the molecular chain of HDPE following irradiation, and the rate and efficiency of HDPE functionalization increased with enhancement of irradiation intensity. After irradiation, the melting temperature, contact angle with water and notched impact strength of HDPE decreased, the degree of crystallinity increased, and their variation amplitude increased with irradiation intensity. Compared with HDPE, the yield strength of HDPE irradiated at lower light intensity (32 W m?2 and 45 W m?2) increases monotonically with irradiation time, and the yield strength of HDPE irradiated at higher light intensity (78 W m?2) increases up to 48 h and then decreased with further increase in irradiation time. The irradiated HDPE behaved as a compatibilizer in HDPE/polycarbonate (PC) blends, and the interface bonding between HDPE and PC was ameliorated. After adding 20 wt% HDPE irradiated at 78 W m?2 irradiation intensity for 24 h to HDPE/PC blends, the tensile yield strength and notched Izod impact strength of the blend were increased from 26.3 MPa and 51 J m?1 to 30.2 MPa and 158 J m?1, respectively. Copyright © 2003 Society of Chemical Industry 相似文献
2.
Titanium Dioxide, TiO2, is a photocatalyst with a unique characteristic. A surface coated with TiO2 exhibits an extremely high affinity for water when exposed to UV light and the contact angle decreases nearly to zero. Inversely, the contact angle increases when the surface is shielded from UV. This superhydrophilic nature gives a self-cleaning effect to the coated surface and has already been applied to some construction materials, car coatings and so on. We applied this property to the enhancement of boiling heat transfer. An experiment involving the pool boiling of pure water has been performed to make clear the effect of high wettability on heat transfer characteristics. The heat transfer surface is a vertical copper cylinder of 17 mm in diameter and the measurement has been done at saturated temperature and in a steady state. Both TiO2-coated and non-coated surfaces were used for comparison. In the case of the TiO2-coated surface, it is exposed to UV light for a few hours before experiment and it is found that the maximum heat flux (CHF) is about two times larger than that of the uncoated surface. The temperature at minimum heat flux (MHF) for the superhydrophilic surface is higher by 100 K than that for the normal one. The superhydrophilic surface can be an ideal heat transfer surface. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
3.
Simultaneous HPLC quantification of total cholesterol,tocopherols and β-carotene in Barrosã-PDO veal 总被引:1,自引:0,他引:1
José A. Mestre Prates Mário A. Gonçalves Quaresma Rui J. Branquinho Bessa Carlos M.G. Andrade Fontes Cristina M.P. Mateus Alfaia 《Food chemistry》2006
A simple, rapid and sensitive procedure for the simultaneous determination of total cholesterol, tocopherols and β-carotene in meat is described. The method involves a direct saponification of the meat, a single n-hexane extraction and the analysis of the extracted compounds by normal-phase HPLC, using fluorescence (tocopherols) and UV–Vis photodiode array (cholesterol and β-carotene) detections in tandem. Rates of recovery of spiked meat samples were 93% for cholesterol, 83–86% for (α-, β- and -γ) tocopherols and 89% for β-carotene. Repeatabilities were high (CV < 6%) for all determined compounds, except for δ-tocopherol. This tocopherol, which is not usually present in meat, showed a much lower recovery percentage (73%) and repeatability (12.8%). This methodology was applied for the quantification of total cholesterol, tocopherols and β-carotene in three muscles (longissimus thoracis, longissimus lumborum and semitendinosus) of the Portuguese traditional Barrosã-PDO veal, obtained from autochthonous calves fed extensively during summer (with the least abundant green pastures) and slaughtered in early autumn (October). Barrosã-PDO veal showed median contents of total cholesterol (0.50–0.56 mg/g) and, depending on the analysed muscle, moderate to high contents of α-tocopherol (3.3–3.9 μg/g) and β-carotene (0.07–0.09 μg/g), suggesting an high sensorial and hygienic quality. 相似文献
4.
5.
Shuko Terazawa Mariko Takada Yoriko Sato Hiroaki Nakajima Genji Imokawa 《International journal of molecular sciences》2021,22(4)
Little is known about the effects on hyaluronan (HA) metabolism of UVA radiation. This study demonstrates that the secretion of HA by human dermal fibroblasts (HDFs) is downregulated by UVA, accompanied by the down- and upregulation of mRNA and protein levels of the HA-synthesizing enzyme (HAS2) and the HA-degrading protein, HYaluronan Binding protein Involved in HA Depolymerization(HYBID), respectively. Signaling analysis revealed that the exposure distinctly elicits activation of the p38/MSK1/CREB/c-Fos/AP-1 axis, the JNK/c-Jun axis, and the p38/ATF-2 axis, but downregulates the phosphorylation of NF-kB and JAK/STAT3. A signal inhibition study demonstrated that the inhibition of p38 significantly abrogates the UVA-accentuated mRNA level of HYBID. Furthermore, the inhibition of STAT3 significantly downregulates the level of HAS2 mRNA in non-UVA exposed HDFs. Analysis using siRNAs demonstrated that transfection of ATF-2 siRNA but not c-Fos siRNA abrogates the increased protein level of HYBID in UVA-exposed HDFs. An inhibitor of protein tyrosine phosphatase but not of protein serine/threonine phosphatase restored the diminished phosphorylation level of STAT3 at Tyr 705, accompanied by a significant abolishing effect on the decreased mRNA expression level of HAS2. Silencing with a protein tyrosine phosphatase PTP-Meg2 siRNA revealed that it abrogates the decreased phosphorylation of STAT3 at Tyr 705 in UVA-exposed HDFs. These findings suggest that the UVA-induced decrease in HA secretion by HDFs is attributable to the down- and upregulation of HAS2 and HYBID expression, respectively, changes that are mainly ascribed to the inactivated signaling of the STAT3 axis due to the activated tyrosine protein phosphatase PTP-Meg2 and the activated signaling of the p38/ATF2 axis, respectively. 相似文献
6.
7.
日光照射对荧光增白剂VBL顺-反异构现象影响的研究 总被引:2,自引:1,他引:2
用紫外光谱和荧光光谱法对日光照射荧光增白剂VBL溶液进行了研究。结果表明、随日光照射时间的延长,VBL溶液在432nm处荧光强度下降;而在267nm处有所增强,低浓度溶液对日光照射更加敏感。1mg/1在避光时测得吸光度为0.449,而在日光照射15分钟后,其吸光度下降至0.141,证明日光照射使VBL溶液中的顺式异构体增加。 相似文献
8.
以水杨酸和间苯二酚为原料,无水氯化锌为催化剂,氯苯为溶剂,通过Friedel-Crafts反应合成了紫外线吸收剂2,2’,4-三羟基二苯甲酮。考察了反应温度、时间、原料配比对产品收率的影响。结果表明:在n(间苯二酚)∶n(水杨酸)=1∶1.2、反应温度120℃、反应时间3h的条件下,收率90%;产物经红外光谱、紫外光谱进行了表征,证明为二苯甲酮类物质,可用做紫外线吸收剂。 相似文献
9.
10.
Chemical properties of femoral gland secretions in the desert iguana,Dipsosaurus dorsalis 总被引:4,自引:0,他引:4
Allison C. Alberts 《Journal of chemical ecology》1990,16(1):13-25
This study investigates the chemistry of femoral gland secretions in the desert iguana,Dipsosaurus dorsalis (Lacertilia: Iguanidae), and discusses their possible functional significance. Electrophoretic and proton NMR studies indicated that the secretions are composed of approximately 80 % protein and 20% lipid material. Individual differences in polyacrylamide gel banding patterns of femoral gland proteins were found. Reflectance spectroscopy revealed that the secretions strongly absorb longwave ultraviolet light, a feature that may be important in the localization of secretion deposits in the environment. 相似文献